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Kate was admitted to the King Edward VII Hospital with hyperemesis gravidarum, a very acute form of morning sickness. It is understood that the pregnancy has not passed the 12-week point and the announcement was prompted by Kate's medical condition.

William was by Kate's side as she went to hospital, leaving last night. Kate is expected to remain in hospital for the next few days.

For women with hyperemesis gravidarum, vomiting can be so severe that they cannot keep food or liquid down. The condition usually continues past the first three months of pregnancy, although it can pass by week 21.

While there is concern for Kate's health, the news that the royal couple are set to be parents was greeted with joy and excitement globally.

Prime Minister David Cameron said it was "wonderful news", adding: "I'm sure everyone around the country will be celebrating with them tonight."

Heir to the throne

The Queen and Duke of Edinburgh, the Prince of Wales and members of both families are delighted with the news, said a St James's Palace spokesman.

Jay Carney, press secretary to US president Barack Obama, said: "On behalf of everyone here in the White House, beginning with the president and the first lady, we extend our congratulations to the Duke and Duchess of Cambridge on the welcome news this morning out of London that they are expecting their first child."

Asked if Mr Obama had any advice as parents, he said: "I haven't had that conversation with them, but I know they both feel that having a child is one of the most wonderful parts of their lives.

"So I'm sure that will be the same for the Duke and Duchess of Cambridge."

Julia Gillard, prime minister of Australia, and John Key, New Zealand's prime minister, also sent their congratulations.

Bookmaker William Hill is already taking bets on the potential name for the baby - with Frances and John at 9/1, and Charles, Victoria and George at 10/1.